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ME 408 LECTURE-2

MODELLING

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Modeling Build the model

Modeling considerations

Element Type

Real Constants

Material Properties

Sections

Geometry/Modeling WorkPlane& Coordinate systems

Keypoints

Lines Areas

Volumes

Meshing

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As you begin your model generation, you will (consciously orunconsciously) make a number of decisions that determine howyou will mathematically simulate the physical system: What are the objectives of your analysis?

Will you need to vary/modify model data?

Will you need to change the geometric topology of the model, e.g. add

holes to the model? Will you model all, or just a portion, of the physical system?

How much detail will you include in your model?

What kinds of elements will you use? How dense should your finiteelement mesh be?

In general, you will attempt to balance computational expense(CPU time, etc.) against precision of results as you answerthese questions.

The decisions you make in the planning stage of your analysiswill largely govern the success or failure of your analysis efforts.


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Linear or Higher Order Elements

Take Advantage of Symmetry The axis of symmetry mustcoincide with the global Cartesian Y-axis.

Negative nodal X-coordinates are not permitted.

The global Cartesian Y-direction represents the axial direction, the globalCartesian X-direction represents the radial direction, and the globalCartesian Z-direction corresponds to the circumferential direction.

Your model should be assembled using appropriate element types: For axisymmetric models, use applicable 2-D solids with KEYOPT(3) = 1,

and/or axisymmetric shells. In addition, various link, contact, combination,and surface elements can be included in a model that also contains

axisymmetric solids or shells. (The program will not realize that these "other"elements are axisymmetric unless axisymmetric solids or shells are present.)

How Much Detail to Include

Appropriate Mesh Density


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Characterization of problem

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The ANSYS program does not assume a

system of units for your analysis.

Units must however be consistent for all

input data.

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BEAM MESH

CIRCUit Multi-Point Constraint

COMBINation PIPE

CONTACt PLANE

FLUID PRETS (Pretension)HF (High SHELL

Frequency) SOLID

HYPERelastic SOURCe

INFINite SURFaceINTERface TARGEt

LINK TRANSducer

MASS MATRIX

VISCOelastic (or viscoplastic)

Element Type

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Main Menu> Preprocessor> Element Type> Add/Edit/Delete

Element Type

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The ANSYS element library contains

more than 150 different element types

Each element type has a unique number and

a prefix that identifies the element category ET,1,BEAM4

ET,2,SHELL63

Element Type

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Many element types have additional options,

known as KEYOPTs, and are referred to as

KEYOPT(1), KEYOPT(2), etc. e.g.:

KEYOPT(9) forBEAM4 allows you to chooseresults to be calculated at intermediate locations

on each element

KEYOPT(3) forSHELL63 allows you to suppressextra displacement shapes

Element Type

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Real Constants

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Real Constants

Element real constants are properties thatdepend on the element type, such as cross-sectional properties of a beam element e.g. real constants forBEAM3, the 2-D beam element,

are area (AREA), moment of inertia (IZZ), height(HEIGHT), shear deflection constant (SHEARZ), initialstrain (ISTRN), and added mass per unit length(ADDMAS).

Not all element types require real constants, anddifferent elements of the same type may havedifferent real constant values.

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Real Constants

For line and area elements that require geometry data(cross-sectional area, thickness, diameter, etc.) to bespecified as real constants, you can verify the inputgraphically by using the following commands in the order

shown:Utility Menu> PlotCtrls> Style> Size and ShapeUtility Menu> Plot> Elements

ANSYS displays the elements as solid elements, using a

rectangular cross-section for link and shell elements and acircular cross-section for pipe elements. The cross-sectionproportions are determined from the real constant values.

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Sections

Building a model using BEAM44, BEAM188, or

BEAM189, you can use the section commands

(SECTYPE, SECDATA, etc.) or their GUI path

equivalents to define and use cross sections in your

models.

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Sections

A cross section defines the geometry of the

beam in a plane perpendicular to the beam axial

direction. ANSYS supplies a library of eleven

commonly-used beam cross section shapes,and permits user-defined cross section shapes.

When a cross section is defined, ANSYS builds

a numeric model using a nine node cell for

determining the properties (Iyy, Izz, etc.) of thesection and for the solution to the Poisson's

equation for torsional behaviour.

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Sections

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Geometry/Modeling

Creating a solid model within ANSYS.

Using direct generation.

Importing a model created in a computeraided design (CAD) system.

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Coordinate Systems

Globaland localcoordinate systems are used to locategeometry items (nodes, keypoints, etc.) in space.

The displaycoordinate system determines the system inwhich geometry items are listed or displayed.

The nodalcoordinate system defines the degree of freedom

directions at each node and the orientation of nodal resultsdata.

The elementcoordinate system determines the orientation ofmaterial properties and element results data.

The results coordinate system is used to transform nodal or

element results data to a particular coordinate system forlistings, displays, or general postprocessingoperations(POST1).

The working plane, which is separate from the coordinatesystems discussed in this chapter, is used to locategeometric primitives during the modeling process.

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Coordinate Systems

(a) Cartesian (X, Y, Z components)

coordinate system 0 (C.S.0)

(b) Cylindrical(R, , Z components)


(c) Spherical(R, , components)


(d) Cylindrical(R, , Y components)


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Geometry/Modeling

Creategeometrical entities

Operateperform Boolean operations

Move / Modifymove or modify geometrical entities

Copycopygeometrical entities

Deletegeometrical entities

Update Geomupdate the geometry in

relation to for example buckling analysis

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Modeling - Create

The hierarchy of modeling entities is as listed

below: Elements (and Element Loads)

Nodes (and Nodal Loads)

Volumes (and Solid-Model Body Loads)

Areas (and Solid-Model Surface Loads)

Lines (and Solid-Model Line Loads) Keypoints (and Solid-Model Point Loads)

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It is a good idea to use keypoints as

reference points in the modeling phase

Create Keypoints (In Active CS)

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Mesh Attributes

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Meshing Size Controls

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MP,EX,1,2E11 ! Young's modulus for material ref. no. 1 is 2E11

MP,NUXY,1,0.3 !Poissons ratio for material ref. no. 1 is 0.3

MP,DENS,1,7800 ! Density for material ref. no. 1 is 7800

Material Properties

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Types of Loads

Structural: displacements, forces, pressures,temperatures (for thermal strain), gravity

Thermal: temperatures, heat flow rates, convections,internal heat generation, infinite surface

Magnetic: magnetic potentials, magnetic flux, magneticcurrent segments, source current density, infinite surface

Electric: electric potentials (voltage), electric current,electric charges, charge densities, infinite surface

Fluid: velocities, pressures

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Types of Loads

Loads are divided into six categories:

DOF constraints

forces (concentrated loads)

surface loads

body loads

inertia loads

coupled field loads

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Load Step

A load step is simply a configuration of loads for

which a solution is obtained. In a linear static or

steady-state analysis, you can use different load

steps to apply different sets of loads -wind loadin the first load step, gravity load in the second

load step, both loads and a different support

condition in the third load step, and so on. In a

transient analysis, multiple load steps applydifferent segments of the load history curve.

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Substep

Substeps are points within a load step at which solutions arecalculated. You use them for different reasons:

In a nonlinear static or steady-state analysis, use substepsto apply the loads gradually so that an accurate solution canbe obtained.

In a linear or nonlinear transient analysis, use substeps tosatisfy transient time integration rules (which usually dictatea minimum integration time step for an accurate solution).

In a harmonic response analysis, use substepsto obtainsolutions at several frequencies within the harmonicfrequency range.

Equilibrium iterations are additional solutions calculated at agiven substepfor convergence purposes. They are iterativecorrections used only in nonlinear analyses (static ortransient), where convergence plays an important role.

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Application of Loads

Most loads are applied either

on the solid model (on keypoints, lines, and

areas) or

on the finite element model (on nodes andelements)

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DOF Constraints

A DOF constraintfixes a degree of freedom (DOF) toa known value. Examples of constraints are specified

displacements and symmetry boundary conditions in a

structural analysis, prescribed temperatures in a

thermal analysis, and flux-parallel boundary conditions

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Forces (Concentrated Loads)

A force is a concentrated load applied at a node in themodel. Examples are forces and moments in a structuralanalysis, heat flow rates in a thermal analysis, andcurrent segments in a magnetic field analysis

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Forces (Surface Loads)

A surface loadis a distributed load applied over asurface. Examples are pressures in a structural analysisand convections and heat fluxes in a thermal analysis

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Body Loads

A body loadis a volumetric or field load. Examples aretemperatures and fluences in a structural analysis, heatgeneration rates in a thermal analysis, and currentdensities in a magnetic field analysis

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Misc. Loads Inertia Loads

Inertia loads are those attributable to the inertia (mass matrix) of a body,such as gravitational acceleration, angular velocity, and angularacceleration. You use them mainly in a structural analysis

Coupled-Field Loads Coupled-field loads are simply a special case of one of the above loads,

where results from one analysis are used as loads in another analysis.For example, you can apply magnetic forces calculated in a magneticfield analysis as force loads in a structural analysis

Axisymmetric Loads and Reactions

Loads to Which the DOF Offers No Resistance If an applied load acts on a DOF which offers no resistance to it (i.e.

perfectly zero stiffness), the ANSYS program ignores the load.

Initial Stress Loading Initial stress loading is only allowed in a static or full transient analysis(the analysis can be linear or nonlinear). Initial stresses can be appliedonly in the first load step of an analysis.

Applying Loads Using TABLE Type Array Parameters

Graphing or Listing the Boundary Condition Functions

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